Current work on mechanisms of intravascular fibrin formation and deposition utilizing the Schwartzman model and I125-labeled rabbit fibrinogen will be continued. Special emphasis will be given to the role of platelets as well as to differences in reactivity between high and lower molecular weight fractions of fibrinogen. The role of non- enzymatic mechanisms of fibrin formation will continue to be examined. These studies will be integrated with work in progress on a potent anticoagulant activity which we have found present in animals whenever thrombin is elaborated. The development of practical methods for the detection of this activity in patients is underway and may serve as another technique for the detection of intravascular thrombin elaboration. The method developed this past year for the separation and quantitation of high and lower molecular weight fibrinogen will be applied to the study of certain patient populations with vascular and thrombotic disorders. It is believed that the special physico-chemical characteristic of high molecular weight fibrinogen may under certain conditions cause it to "precipitate" on the vessel wall.